Most of the stuff of Jetbrains is open source (under Apache Software License) and is available on github.
I know what you mean though: I would never again use a closed-source IDE, or a language with closed-source standard library. Microsoft Visual Studio made me learn that, decades ago.
So yeah - nevermind about CLion! I do think it works better than IntelliJ, but it's a subtle, not-easily-quantifiable thing, and it's probably not worth buying it if you're just dabbling and also have IntelliJ Ultimate.
After using IntelliJ IDEA plugin [1] (works in free community version too), I don't understand what else people need :) Plugin is pretty powerful. Also Racer works great in VS Code and other editors.
> Rust should have a lower learning curve
Rust is easy enough to learn, the only issue is a steep start. After couple of months everything is not harder than JS and smart compiler actually makes me more and more lazy :) So I think only thing we need is books/articles to help people switch from other languages, to help them overstep initial difficulties.
https://github.com/intellij-rust/intellij-rust
The rust tooling is really coming around.